Extensive research is being done worldwide to develop new, commercially viable methods for removing undesirable chemical species from air or from exhaust gases, such as combustion exhausts, contaminated liquids, such as industrial process effluents, or biologically contaminated water, and contaminated surfaces. One of the most prevalent undesirable components in a gas phase (e.g., polluted air) is nitrogen oxides (NOx). A major NOx component is nitric oxide (NO) which is the primary pollutant in all fuel combustion exhaust gases. Nitric Oxide gas oxidizes in the atmosphere to form nitrogen dioxide (NO2). Nitrogen dioxide is the primary reactant in atmospheric, photochemical reactions which produce unhealthful air pollutants, such as ozone. Nitrogen dioxide is also known as an acid gas which, together with sulfur dioxide, causes acid rain.
Major sources of NO emissions are internal combustion engines and utility boilers. Nitric oxide is formed at high temperatures during fuel combustion by the reaction of nitrogen and oxygen gas components of combustion air (i.e., thermal NOx), and the oxidation of fuel bound nitrogen compounds (i.e., fuel NOx).
Among the approaches to addressing NOx reduction are technologies employing electrical excitation in the gas phase, such as electrical discharge and electron beam techniques. In some of these techniques, NO gas has been shown to be reduced in a gas stream whereby xe2x80x9cdeactivatedxe2x80x9d nitrogen atoms generated from, for instance, a plasma jet are injected into the gas stream. However, such plasmas operate to dissociate molecular nitrogen to deactivated nitrogen atoms at relatively high temperatures, e.g., 900 C. Furthermore, in U.S. Pat. Nos. 5,547,651 and 5,782,085, microwave irradiation causing dissociation of molecular nitrogen into nitrogen atoms has been employed in 97-99% pure nitrogen streams for NOx reduction.
The use of electron beam irradiation for flue gas treatment (NOx/SO2 removal) has been conducted as early as 1970. When the electron beam process is used to clean the flue gas from an electric utility boiler, the flue gas is first cleaned of flyash by a particle collector. The gas then passes through an evaporative spray cooler where the gas temperature is lowered, as the humidity is increased. The gas then passes to a process vessel where it is irradiated by a beam of high-energy electrons, in the presence of a near-stoichiometric amount of ammonia that is injected upstream of the process vessel. SO2 and NOx are oxidized to form H2SO4 and HNO3, respectively. These acids subsequently react with the added ammonia to form ammonium sulfate and ammonium sulfatenitrate. Such salts are recovered as a dry powder using a conventional particle collector and the collected powder is useful as an agricultural fertilizer.
Most studies of electron beam flue gas treatment have used ammonia as the reagent, with ammonium sulfate and ammonium nitrate as the major products. The use of alternative reagents has also been studied. The sulfuric and nitric acids can be neutralized by an alkali-slurry spray of hydrated lime in a spray dryer. The products formed are calcium sulfate and calcium nitrate, which are harmless granular solids. This method can be used for the simultaneous removal of SO2, NOx and HCl from the flue gas of waste incinerators.
Although electron beam processing can be very effective, the high capital cost and x-ray hazard associated with its present implementation in power plant applications have discouraged researchers from using this technique in small scale applications, such as treatment of engine exhaust. Conventional accelerators employed in electron beam apparati use a xe2x80x9cwindowxe2x80x9d containing a thin metal foil (e.g., Tixe2x80x94Pb). The foil window forms a barrier between the electron beam source, which is under high vacuum, and the atmosphere in the reaction chamber. It is sufficiently thin so as to be partially transparent to the electron beam. The high accelerating potential necessary to penetrate the foil window requires an expensively constructed electron beam gun having stringent insulation requirements. At the same time, such a high accelerating potential produces strong x-rays at the point of incidence, so that extensive radiation shielding is also required. Conventional windows have a high transmission loss, leading to high usage of electricity and high thermal loading.
Recently, compact low-energy electron accelerators have been developed to meet the requirements of industrial applications, utilizing for example, special materials that make the window thin and rugged, as disclosed for example in U.S. Pat. No. 6,002,202. However, successful application of compact low-energy electron beam accelerators in vehicle exhaust clean-up has not been achieved.
Accordingly , a need exists to improve NOx reduction efficiency using excitation techniques, particularly in exhaust gas streams from vehicle engines.
The invention includes a system, vehicle and process that employs one or more compact electron beam device(s) in the presence of relatively pure nitrogen gas to inject deactivated nitrogen atoms into a NOx-containing engine exhaust to efficiently reduce NOx. A relatively pure (greater than 99%) and essentially oxygen-free nitrogen gas source is preferably adapted for injection into the exhaust at relatively low exhaust temperatures, e.g., less than about 200 degrees C., and usually about 100 degrees C. or less.
Due to such low temperatures during NOx reduction, the invention allows at least a portion of the emissions control equipment of a vehicle to be located remotely from the engine. An advantage of the invention is that it provides highly efficient NOx reduction without the necessity of a catalyst. Furthermore, the non-catalytic process of the invention is effective, particularly for highly efficient NOx reduction (more than 90% NOx reduction) in exhausts of advanced engines that emit an initial NOx exhaust content of 150 ppm or less at temperatures of 200 degrees C. or less. It has been difficult for state-of-the-art catalyst systems to achieve high NOx reduction under these conditions of relatively low initial NOx and low exhaust temperature.